This program seeks an understanding of the interaction of ultrasound with biological material in two broad perspectves: (1) Propagation of Sound in Tissue with particular emphasis on nonlinear phenomena and (2) the Biological Effects of Ultrasound. In the latter work, both thermal and nonthermal mechanisms have been demonstrated. Particularly important are studies of the mechanism of killing of Drosophila at low temporal average intensities of pulsed ultrasound, and implications of these studies for human safety.